A major portion of the services provided by the oil field service industry relate to cementing operations, including primary cementing, that is, cementing casing into a well bore by pumping cement slurry down a centralized casing and up into the annulus between the well bore and the casing, and squeeze cementing of a particular zone or interval. Additionally, cement slurry is often pumped downhole for water control, fluid loss control, and many other purposes.
Another aspect of the oil field service industry is stimulation services which include, among other operations, fracturing an oil bearing formation by pumping a pressurized fluid into well bore perforations until the oil bearing formation fractures. A proppant laden slurry is then pumped down hole after the fracturing fluid. This fracture is then held open by the proppant, usually sand or bauxite, which remains embedded in the walls of the fracture after the fluid contained within the slurry migrates into the surrounding formation, or, ideally, is flowed back into the well bore out of the formation when pressure is reduced.
In all of the aforementioned situations it would be advantageous to have a quick-response system for determining the density of the cement slurry, or the proppant slurry.
In oil well cementing operations, the density of the cementing slurry is an important factor. The bore hole cementing fluid typically is a slurry of chemical constituents mixed with water and has a certain density. Should the composition of the slurry mixture change during the pumping operation, the density changes and a change in mixture can effect desired results in the cementing operation. For that reason, it is desirable to be able to sense changes in density, i.e. changes in the mixture in order to be able to provide a correction to the mixture before a large volume of incorrect mixture is introduced into the system.
Similarly, in fracturing of wells, monitoring of the density of the fracturing fluid, or proppant slurry, is desirable to ensure that there is not too little proppant in the slurry, which can result in fracture closure, or too much proppant, which can result in "sand out", or termination of the operation due to plugging of the pump, lines or well bore with proppant.
It is accordingly, a feature of the present invention to obtain a relatively quick-response time to the change of density in a cementing or fracturing fluid system so that the fluid may be continuously monitored and corrected if necessary to obtain a consistent density for the fluid mixture.
The present invention relates to a method and apparatus for processing randomly varying data to obtain quick response control to changes in the density of a fluid.
The prior art has developed digitally processed data for nuclear densometers, as illustrated by U.S. Pat. No. 3,657,532, issued to Carl W. Zimmerman. As set forth in the `532 patent, digital systems allow the incorporation of reliable, inexpensive and compact integrated circuits and can be used to develop digital pulse counting techniques. However, in this prior art apparatus, there remains a substantially long time response to a change in density in the fluid sample being tested, and as a consequence, a considerable volume of incorrect density fluid may be passed through the system for use before a correction in the density can be detected or made.
U.S. Pat. No. 4,618,939 to Davis and assigned to the assignee of the present invention involves a method and system for sensing the density of a fluid and for providing statistical count signals which are proportional to density. This prior art system only identifies significant changes in density and then responds to these changes only after a substantial period of time.
Additionally, U.S. patent application Ser. No. 262,406, filed Oct. 25, 1988 and assigned to the assignee of the present invention describes a digital filter which can be used with a radioactive densometer. The digital filter uses a weighting factor including two confidence factors to determine whether or not an actual change in state has occurred.